


Underswap Sans has a Good Time

by DeckofDragons



Category: Undertale (Video Game)
Genre: Alternate Universe - Underswap, Consensual Oviposition, Consentacles, Eggs, Friends With Benefits, M/M, Mpreg, Oviposition, Tentacle Porn, friends with lots of benefits, skelepreg
Language: English
Status: Completed
Published: 2018-04-14
Updated: 2018-04-14
Packaged: 2019-04-22 17:24:09
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 3,891
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14313564
Author URL: https://archiveofourown.org/users/DeckofDragons/pseuds/DeckofDragons
Summary: Underswap Sans gets treated a like a Japanese schoolgirl by a tentacle monster in Waterfall.Or Underswap Sans enters into a friends with benefits relationship with a tentacle monster in Waterfall. One of those benefits includes breeding.





	Underswap Sans has a Good Time

**Author's Note:**

> If you can't tell or haven't read 'Swapfell Sans Suffers', this fic is based off that one. I've had the idea to write something like this ever since I first started working on that fic. I finally decided to do it just for fun and as a sort of birthday gift to myself (today, April 14th, is by birthday yay!) because 'Swapfell Sans Suffers' was my first foray into being a porn author. Which is almost all I write these days because it's fun. I'm still not 100% how it happened though, there was a time when I thought I'd never be able to write porn or if I ever did it would be an occasional thing but look at me now. :)

Sans half-skipped half-walked as he followed the river on the way to the Royal Guard’s Head Quarters. Today was the big day, the big test to get into the Guard. He’d been waiting for this moment since he’d started training in earnest to become a Royal Guard. Lots of people were going to be there, only the best of the best would make it and he was for _sure_ going to be among them.

He was the first one there. But that was fine, he could wait. He had left early to show his dedication for this… and because he was too eager to wait around at home. He hadn’t even told Papyrus he was leaving, he was so eager. But Papyrus would be here in time to watch the whole thing, he’d promised, and then he’d congratulate Sans on getting his rightful place in the Guard.

…

“I’m sorry bro,” Papyrus said as they were leaving, walking past huddles of people congratulating their friends and family on making it into the Guard. “I’m sure you’ll get in next time. This is only your first attempt after all, no one gets in on their first attempt.”

Sans sighed, kicking his feet as he walked. “I guess.” He’d been _sure_ that’d he’d get in, but… he wasn’t good enough.

“C’mon, let’s go home.”

“No, I think I want to walk around for a little bit but you can go.” Sans wanted to be alone right now. He’d been bragging to Papyrus for the past few weeks about how he was going to get into Guard today but now he felt like a fool.

Papyrus studied him for a few seconds, a worried expression on his face. Sans _hated_ making him worried, another reason he needed to be alone right now. “Well uh… okay, if ya need me to come pick ya up just call or text or whatever. Or like if ya need anything else.”

“I’ll be back in time to make supper.”

“See ya then… I guess. Just ya know if ya wanna talk ‘bout it, I’m…”

“Just go Pappy, I’ll be fine, I just need sometime to think and unwind.” He put on his best enthusiastic smile. “I’ll get in for sure next time, I _know_ it.”

Papyrus visibly relaxed. “’Kay, see ya later then,” he said before teleporting away.

Blueberry let his face and shoulders slump once more. He’d be fine, he just needed some time. And he’d get in eventually, even if it wasn’t next time. But for now, he just needed to let himself feel disappointed and sad for a little bit.

 

The river burbled soothingly as Sans walked beside it. Sometimes he wished they lived in Waterfall because how lovely and calming the multitude of river and streams were. But Papyrus liked the snow more and had a friend behind the door to Ruins – he didn’t know Sans knew about that – and there was also Muffet. With how bad Papyrus was at making friends Sans couldn’t take that away from him.

The cavern he was walking down was dark and empty. The only light sources were the occasional echo flower, glowing mushroom, or crystal ‘star’ in the ceiling or wall. It was rather peaceful. He’d have to come back here sometime to enjoy it when he was in a less melancholy mood.

“You look sad.”

Sans jumped, almost stumbling over himself and falling forward. He looked over to see something poking out of the water. Something round with large eyes too far back in the shadows to get a clear view of. Another monster, Sans hadn’t been as alone as he’d thought.

Most of the other monster’s body was hidden below the water, dark and seemingly bottomless, so there was no telling how big they were but they gave off the sense of being rather large.

Sans brushed himself off, trying to pretend he hadn’t been startled as he smiled at the monster in the water. He was a bit nervous, he was alone with this unknown monster in a dark cavern after all. That had the potential to be dangerous but they were clearly water bound and thus wouldn’t be able to chase him if he ran. “I’m Sans, who are you?” Introductions were a good way to start things if this was a friendly monster and if they weren’t they’d perhaps be put off being dangerous by his lack of outward fear or nervousness.

“Onionsan.” He drifted a little bit closer, enough that it became clear he was a giant octopus monster. Or if this were an anime, a tentacle monster. … Gosh, had Sans really just had that thought? This was a real-life monster not a hentai one. He didn’t need to be thinking of Onionsan’s tentacles like that even if he did like tentacles. “But why did you look so sad?”

Sans’ let his happy expression drop. Onionsan was a stranger and therefore Sans didn’t have to worry about pretending to be happy for him. And since he didn’t seem to have any ill intent there was no need to fake it for protection purposes either. “I didn’t make it into the Guard like I thought I was going to.”

“Oh, I’m sorry.” Onionsan’s eyes shifted back and forth as he flopped the tip of a tentacle back and forth just above the water’s surface. “Uh… why not? If you don’t me asking?”

“I wasn’t good enough to pass all the tests.” He sighed as he kicked a random pebble into the river. “It’s okay though, there’s always next time. This was only my first try after all. I’m just… disappointed because I thought I’d get in and now I feel like a fool for believing it. I really shouldn’t be as upset as I am about it.” There were people who had much worse problems out there.

“It’s okay to be sad sometimes. I’m sad too.”

“Why are you sad?” They could exchange tales of woe, that wouldn’t hurt.

Onionsan seemed to almost blush though it was hard to tell in the darkness. “I’m lonely.”

“I can keep you company if you like.” Sans was good at that and making new friends was always fun. And it would make this day not a total failure.

“Oh uh… okay.”

Sans sat on the ground in front him, legs crossed. He didn’t bother forcing himself to act happy, he’d already given his true emotions away. And it’s not like his brother was here to worry about him.

 

After that meeting with Onionsan became a regular thing. It wasn’t all too different from Papyrus’ meetings with the guy behind the Ruins’ door, just a bit further out of the way and therefore less frequent. But he made a point to come at least once a week.

Initially he had no intention of keeping a secret from Papyrus, there was no reason to after all. But it never came up, just like the Ruins’ guy never came up either, Sans only knew about _him_ because he’d seen Papyrus chatting with him several times – unnoticed because of how focused Papyrus always was on the conversation he was having, enough that Sans didn’t want to disturb them.

Then Sans found a reason to keep it a secret; Onionsan not only knew what hentai was and how it often included tentacle monsters but also liked it at least as much as Sans did. And well he _was_ a tentacle monster, with Sans’ kinks leaning heavily in that direction, the topic of friends with benefits was bound to come up eventually. And once the topic was breached it wasn’t long before they engaging in that kind of relationship, making him Sans’ second sexual partner.

“Uh… Sans, we’re pals right?” Onionsan said one day while they were basking in the afterglow.

“Of course we were.” Sans was still naked but kept warm by Onionsan’s tentacles holding onto him, gently rocking him back and forth. After sex was when he felt the most calm, not like he needed to be doing everything that needed to be done as soon as possible. It was a nice change.

“Well uh… if I asked you to help me with something, would you?”

“That depends on what it is but probably, if it’s something I can actually help with.” Sans liked helping people, it was nice making others happy. “If not, I could try to give you advice on what to do.”

“It is definitely something you can help with.”

“Cool, what is it?”

Onionsan blushed, retracting under water a bit more. “Uhm… actually it’s nothing.”

“It’s clearly not nothing.” Sans smiled wide at him, the one he used to encourage people to come out of their shells. “You can tell me.”

“Well… as I’m sure you may have noticed, I’m uh… the only monster type of my kind left. Everyone else died.”

“Oh, I’m sorry.” Sans had been wondering about that and had been hoping he’d just never seen Onionsan’s monster type because it was a rare one not because he was the only one.

“It’s… whatever” Onionsan lifted his tentacles in a shrugging gesture. “It happened ages ago. But… my kind doesn’t have to go extinct. I can… lay eggs in almost any other monster type and perpetuate my species that way.” He blushed even more, sinking further in the water and having to lift Sans a little more so he wouldn’t get wet too.

“Are you asking me to… carry your eggs or something?” Sans said, rolling his head around in an attempt to make eye contact. What else could it be?

Onionsan hesitated for several long seconds before raising again so his mouth would be out of the water. “Yes, but uh… if you don’t want to that’s fine.”

It was certainly an interesting idea and Onionsan’s species would go extinct without this but… did Sans was want to carry his eggs? There’d probably be no hiding it from Papyrus if he did, unless they were small. “I think I need some time think about it.”

Onionsan stiffened as if surprised by that. “Of course you can think about it, no worries.”

Sans would have to talk about it with him more too, get the specifics of everything, including possible risks. But for now, he’d enjoy hanging out here for a little bit longer before moving on to training.

 

“How long would it take them to hatch?” Sans asked, sitting crossed leg on the ground in front of where Onionsan floated in the water.

“About three months,” Onionsan replied. They’d been discussing this since Sans had gotten here. He’d been weighing the pros and cons of this for the past week all he needed was a few more specifics and now he had all that so it was time to make his decision.

Three months wasn’t too bad. He’d be fat though and pregnant so he’d have to cut back on training and wouldn’t be able to hide it from Papyrus, who _still_ knew nothing about all this. Sans had considered telling him about it but had been afraid of how he might react. Would be mad at Sans or encourage him do it if he really wanted to? Would he disapprove and insist he didn’t do it and perhaps try to get him to stop having casual sex? He was Sans’ brother so Sans should have at least some idea, right? But… Sans’ sex life had never come up in conversation or in any other way, in fact, Sans had very specifically hidden that stuff from him for fear of his reaction to it. He probably should stop doing that – should’ve never done it in the first place – now might be a good time for it.

And if nobody did this Onionsan would be alone forever, the last of his kind. It would not only be the right thing to do but whoever did it would also be a hero in a way. And right now it seemed like Sans was his only friend because of how far out of the way his cavern was and there was no way for him to move to somewhere else because he was water bound.

“I’ll do it then.” _And_ Sans had seen oviposition porn in the past and was more than okay with it. So, really it was just a continuation of their friends with benefits relationship, he was just offering a slightly different kind of benefit. And Onionsan would be the primary caretaker of the babies once they hatched, meaning he wouldn’t have to worry about it.

“Oh, okay,” Onionsan said, sounding surprised. “Are you sure.”

“Yeah, let’s do it.” Sans stood and started undressing so his clothes wouldn’t get wet during this. Once done he folded them neatly and placed them back a bit, next to the big fluffy towel he always brought with him when he came out here.

“You mean right now?”

“I don’t see why not unless you’re not ready yet,” Sans said as he settled himself back on the ground naked. He scooched forward to put his legs in the water. It was a little chilly but not necessarily unpleasantly so.

“No, I’m ready, have been for a while now, it’s… quite uncomfortable.”

“Then let’s do it now.”

Onionsan hesitated for a second or two longer before his tentacles rose from the water to wrap around Sans. They’d had sex enough times by now that he knew right where to touch and stroke him to make him gasp and writhe in pleasure.

“The sounds you make are always so cute,” Onionsan said with a smile. It was interesting how ‘cute’ could mean entirely different things depending on the context, Sans _far_ preferred this one being applied to him over what people normally meant when they called him ‘cute’.

His magic was already pooling in his pelvis, its warmth pleasant in pleasant contrast to the cool surroundings. Another tentacle moved to stimulate it and it morphed into a pussy as he bucked up into. The tentacle then pushed into him, making him gasp as his magic stretched around it.

“Why aren’t you summoning your own magic?” Sans panted out as Onionsan’s pumped into him, starting gentle and steady.

“I’m going to be putting eggs into you, remember? So I can’t.” Onionsan’s other tentacles tightened a bit around Sans as the one he was fucking him with gradually sped up. “I’m going to make you come first though because it’ll make it easier for both of us.”

Sans nodded as he relaxed back into Onionsan’s hold, clenching his magic around the tentacle as it filled him. Onionsan’s tentacles rubbed and caressed him, one wrapping around his spine and stroking it in time with his thrusts. Sans arched into it with a gasping moan.

Onionsan had built up the intensity a lot quicker than he normally did so it wasn’t long before Sans’ orgasm starting building. Sans had gotten louder as he always did – he had trouble keeping quiet even when he was trying to – so thank goodness this cavern was out of the way. So when he climaxed was with a sound almost loud enough to echo, waves of pleasure rolling through him, the thrusting tentacle helping him ride it out and making it longer.

As he relaxed, it stilled and pulled out. It was odd without Onionsan coming inside him, that always felt good. But it made sense here, if Onionsan was going to be putting eggs into him then having cum in there wouldn’t help.

“You really sure you want to do this?” Onionsan asked.

“Yes.” Blueberry smiled with the enthusiasm he could, which was a lot. His body was still burning with arousal, not quite done yet. “What do you need me to do, if anything?”

“Expand your magic into a womb to hold the eggs.”

Sans had never done that before but all he had to do was try and his magic did the rest based off his intent even though it took several seconds to form. His magic formed into an abdomen too, to protect that new organ. He bent to look at it. It was blue and see through, meaning the eggs would be visible inside him, an interesting prospect.

He looked back up as another tentacle rose out of the water. It was slightly thinner than the others and looked hollow and transparent, tapering off into a closed opening. It made its way between his legs and pushed into his pussy, sliding in with ease. It stopped pushing deeper in when it was nearing his womb. Magic tendrils grew off where it entered and latched onto his pussy, securing it in place.

“Is this your ovipositor?” Sans asked, squirming a little. It wasn’t super comfortable the way it gripped onto his magic, though it wasn’t too bad either.

“Yes,” Onionsan said. “So, I guess this is your last chance to change your mind about this.”

“Nah, I want to help. So, egg me Daddy.” He shaped his hands into finger guns and winked.

“Well, I guess I am going to be a daddy, huh?” Onionsan said thoughtfully. Oops, Sans hadn’t quite been thinking along those lines; he’d been trying to be funny but now he was just embarrassed instead. “I wonder if they’ll call me ‘daddy’, I hope so. Anyway, you ready? I can’t… take this much longer.”

“Yeah, I’m ready.” And grateful that Onionsan evidently didn’t notice his embarrassment at his attempt at a joke.

“Very good.” Onionsan smiled bright, a cute expression on his round face.

After a short pause a round bulge appeared in the ovipositor, rising out of the water. It looked a bit like a giant fish egg. Which was kind of gross but better than hard eggs, those probably would’ve hurt or at least have been super uncomfortable.

It traveled down the ovipositor until it reached his opening. He gasped as it pushed in stretching him; it was thicker than Onionsan’s tentacles were – good thing he’d climaxed before or it’d probably be painful. When it reached the end of the ovipositor it pushed out into his magic, making him jerk instinctively. It was soft and squishy with a jello-like consistency. It was an… interesting sensation that was over after a few seconds when it slid into his womb.

He bent to look at it inside him. It looked like a ball inside his belly, seemingly blue though his blue ecto-flesh. “How many are there?”

“Hmm… I don’t actually know, they’re kind of hard to count when they’re inside me. But I don’t think it’s going to be anywhere even close to enough to cause you any pain or anything.” That was good.

There were more eggs making their way through the tube as he looked up once more. One was just reaching him; he watched it push through and then into him, making him shiver a little at the sensation before it settled in his womb with the first one.

He kept watching them, fascinated by the sight. It was almost watching one of those ‘Most Satisfying Video in the Underground’ videos except in real life. And it was accompanied by the feel of them in his magic, along with a slowly growing weight in his belly.

“They’re super cute,” Onionsan said, rubbing a tentacle of Sans’ middle. Enough eggs were inside it now to completely fill it and it was starting to bulge out, making the eggs more visible as they pressed up against the walls of their new home.

Sans wouldn’t use the word ‘cute’ to describe them but maybe interesting, in a good way. By now he was used to the jello-like feel of them as they passed through magic before slipping into his belly. And with his arousal this was starting to become more than a little hot, each one more so than the last.

By the time his belly was starting to feel heavy with them, he moaned as one pushed in, triggering an orgasm to roll through him. He panted as he came down, his body still ready for more even if a short rest was starting to sound nice.

“Oh, it feels good?” Onionsan asked. His eyes remained glued to Sans’ middle though, his tentacle petting over and massaging it, a _wonderful_ feeling.

“Yeah, kinda,” Sans said as another egg settled his womb, making him that much bigger. Oh, Papyrus was going to be _so_ surprised when he saw Sans like this… so would everyone else since he’d neglected to tell anyone. He probably should have but… oh well, it was his body he could do with it what he wanted.

He could almost feel the added weight of each new egg as they pushed through his magic to his womb. It wasn’t long before he was uncomfortably heavy and he couldn’t see past his belly to his magic anymore no matter how hard he tried. But he climaxed again, this one leaving him feeling like he was satisfied, another one would start to become too much.

“We’re almost done,” Onionsan said, still massaging Sans’ middle, chasing away the ache of having it stretch so much so fast. “Five left.”

Sans let out a sigh of relief. He could _easily_ take that without it becoming too much. Unable to watch, he tilted his head back and counted them as they entered, pushing into him, then his magic before settling his womb. The last one seemed to take longer, thought that was probably because it was final one, but it was soon settled inside him too.

“All done,” Onionsan said, his voice bright and cheery, bringing a content smile to Sans’ face. “Thank you, very, _very_ much.”

“Anytime, it was fun,” Sans said as the tendril holding the ovipositor in place inside his pussy dissipated. It then withdrew with a wet sucking sound because of how much excess magic his climaxing had produced and disappeared under the water.

Sans dispelled his magic and his womb automatically sealed itself to keep the eggs safe inside him. He put a hand over his belly, rubbing it over from the top to the bottom. It was bigger than he’d thought it be, if he wore a shirt over it everyone would think he was due to give birth any day. He wouldn’t be getting a whole lot done over the next three months, he’d already accepted that anyway though.

“Now… are you ready to go home or do you want to rest here for a little bit?” Onionsan asked. His tentacles were still petting over Sans’ belly and now his back and shoulders too.

Sans relaxed into his touch. “I think I’ll rest here for a bit.” He was too tired to move and needed some time to get used to the new weight in his middle. And Onionsan probably wanted some time to appreciate his eggs so Sans should let him.

Onionsan’s tentacles scooped him up to cradle him in the air, one still massaging his belly, another his shoulders – tentacles had so many wonderful uses. “You going to take a nap?”

“No, it’s the middle of day.” Sans eyes were drooping though. Onionsan’s gentle rocking wasn’t helping. “I just… need some… rest.” His head drooped too. Okay maybe he could take a quick nap, there was nothing wrong with that, was there?


End file.
